Because most of the Y chromosome is passed unchanged from father to son, apart from occasional mutations, DNA analysis of the Y chromosome can reveal whether or not individuals are likely to be male-line relatives. We therefore analysed DNA from the Y chromosomes of: five male-line descendants of two sons of the president's paternal uncle, Field Jefferson; five male-line descendants of two sons of Thomas Woodson; one male-line descendant of Eston Hemings Jefferson; and three male-line descendants of three sons of John Carr, grandfather of Samuel and Peter Carr (Fig. 1a). No Y-chromosome data were available from male-line descendants of President Thomas Jefferson because he had no surviving sons.
Seven bi-allelic markers (refs 7–12), eleven microsatellites ( ref. 13) and the mini-satellite MSY1 (ref. 14) were analysed (Fig. 1b). Four of the five descendants of Field Jefferson shared the same haplotype at all loci, and the fifth differed by only a single unit at one microsatellite locus, probably a mutation. This haplotype is rare in the population, where the average frequency of a microsatellite haplotype is about 1.5 per cent. Indeed, it has never been observed outside the Jefferson family, and it has not been found in 670 European men (more than 1,200 worldwide) typed with the microsatellites or 308 European men (690 worldwide) typed with MSY1.
Four of the five male-line descendants of Thomas Woodson shared a haplotype (with one MSY1 variant) that was not similar to the Y chromosome of Field Jefferson but was characteristic of Europeans. The fifth Wood-son descendant had an entirely different haplotype, most often seen in sub-Saharan Africans, which indicates illegitimacy in the line after individual W42. In contrast, the descendant of Eston Hemings Jefferson did have the Field Jefferson haplotype. The haplotypes of two of the descendants of John Carr were identical; the third differed by one step at one microsatellite locus and by one step in the MSY1 code. The Carr haplotypes differed markedly from those of the descendants of Field Jefferson.
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